1. Field of Invention.
Generally the present invention relates to a discharge lamp which could be utilized for the purpose of automotive lighting applications. More particularly, the present invention relates to an electrode assembly suitable for employment in a low-pressure discharge lamp to produce a glow discharge therein, and method of attachment therefor. More particularly, the present invention relates to such an electrode assembly which is suitable for low-pressure discharge lamps comprising borosilicate glass, having a coefficient of expansion (CTE) of between 32-45xc3x9710xe2x88x927/xc2x0C.
2. Description of Related Art.
Typically, in a neon discharge lamp the electrode embodiment is located within the glass tubing and consists of a metal shell coated with an emissive coating. A connection to an external power source is made via a wire which is glass-to-metal sealed in the tubing. In order to ensure an hermetic glass-to-metal seal the metal wire must be compatible with the base glass, i.e., the coefficients of thermal expansion of the respective materials must closely match. Otherwise, mismatch in the respective coefficients of thermal expansion will yield a weak seal which will most likely leak and eventually lead to failure of the discharge lamp.
The traditional neon industry uses soft glass, such as soda lime glass or lead glass, which finds a near perfect expansion match in dumet wire, a nickel-iron alloy with a copper sheathing. However, for borosilicate glass the situation is less clear. Borosilicate glass, also known in the art as hard glass, is preferred for its good thermal properties, in particular good thermal shock resistance, resistance to weathering and low coefficient of expansion, which is typically about 35xc3x9710xe2x88x927/xc2x0C. The closest expansion match to borosilicate glass for any practical metal is tungsten, which has a CTE of 44xc3x9710xe2x88x927/xc2x0C. However, tungsten electrodes are expensive and difficult to manufacture due to the brittle nature of the metal. In addition borosilicate glass-to-tungsten seals are known to be notoriously leaky and often require a multiple glass seal between the borosilicate glass and tungsten metal. Nevertheless, tungsten is used because the alternatives are few.
One method to overcome glass-to-metal expansion mismatch and attain an hermetic seal is by using, what is known in the art as a Housekeeper seal. More information relative to the Housekeeper seal can be obtained, if such information is desired, by reference to U.S. Pat. No. 1,293,441 (Housekeeper). In particular, the Housekeeper seal is employed in the art of sealed beam headlamps made from borosilicate glass to join metal components, known as ferrules, to the reflector of the lamp. Ferrules act as sites for attachment of the incandescent filaments, and function to electrically connect the headlamp to external power circuitry. The ferrule is usually in the form of a hollow, stepped metal capsule closed at one end and open at the other end, the open end being joined to the glass reflector. If more information is desired relative to the use of ferrules in sealed beam headlamps, such information may be obtained by reference to U.S. Pat. No. 4,339,685 (Saguchi et al.) and U.S. Pat. No. 4,479,072 (Gaugel et al.).
The present invention employs the basic concept of the ferrule Housekeeper seal as a starting platform for the construction of an electrode assembly that provides a robust and relatively inexpensive glass-to-metal seal in low pressure borosilicate glass discharge lamps.
U.S. Pat. No. 5,387,837 (Roelevink et al.) discloses a low-pressure borosilicate glass discharge lamp comprising a cylindrically curved metal tube fused to a glass lamp, wherein the metal tube acts as the electrode, and comprises 29% by weight Ni, 17% by weight of Co, remainder Fe, or an Ni/Fe alloy. Even though a functional electrode for borosilicate glass is disclosed therein, in practice it is associated with a very high cost, inasmuch as a gradient seal would be required at the glass-to-metal interface.
An object of the present invention is to provide an electrode assembly having a robust borosilicate glass-to-metal seal and attachment method therefor, whereby the electrode assembly is also suitable for use in a low-pressure discharge lamp.
Another object of the present invention is to provide an electrode assembly having a long-life.
Still a further object is to provide an electrode assembly that is suited for high-volume production and is cost-efficient.
According to the present invention, these and other objects are achieved in the invention described herein. Briefly, the present invention resides in an electrode assembly having a robust seal at the metal and glass interface, and a method of attachment therefor to a low-pressure discharge lamp.